You can block an IP by going to yoursite.com/ip/1.2.3.4 and clicking the button.
If you are looking at the IPs in your database you can just check the "handle" field then go to the user page at yoursite.com/user/handle and it will show you the IPs, which you can click to go to the same IP page as above.
Or, use the MySQL function INET_NTOA to get readable IPs, e.g. "SELECT INET_NTOA(createip) FROM qa_users"